I don't use B4i but in B4A #Minversion is the earliest version of Android the app supports, so I assume it's the same in B4i. If you set it to 12.1 your app will only run on devices running iOS 12.1 & later. I generally support iOS 8.x & up, but it depends on what features are in your app & what is the earliest SDK they are supported in.
Note that the Xcode version & #Minversion aren't related. You can build an app using the 12.1 SDK, but it can still run on devices running earlier versions of iOS, depending on what you set #Minversion to.
- Colin.